1. Field of the Invention
This invention relates to resistor structures in general and more particularly to a novel and improved resistor grid assembly and resistor grid element capable of extremely high wattages for its size.
2. Description of the Prior Art
The present invention relates to resistor structures and in particular to elements of the resistor structure which allow the resistor to maintain extremely high wattages.
There are essentially three types of resistors available today in the industry. The low current resistor carries a nine to 100 amp current. This type of resistor controls smaller AC/DC motors.
The intermediate resistor can handle a current of 100 to 200 amps and is suitable for somewhat larger motors. Higher horsepower motors as employed in mass transmit cars need high current resistors which can carry currents of 200 amps or more and which are rates at 750-900 volts.
High capacity resistors carry currents of over 200 amps. Most high capacity resistors available today are rated at 600 volts without standoff insulators. The present invention described herein is rates at 1000 volts without standoff insulators.
In the past, some high current resistors have been designed by combining numerous intermediate resistors. This results in the obvious disadvantage of a larger size requiring more space and support for the heavier weight. In addition, multiple current paths increase the possibility of overloads and failure. Maintaining a single current path reduces the potential for overloads and failures.
High capacity resistor grids have typically been arranged in the form of a series of parallel metal grid elements affixed to and positioned between insulating members. When high current values are shunted into the resistor grids, temperatures elevate to such a point that warpage or melting could occur if the grids are not effectively cooled. Therefore, it is necessary to maintain air spaces between the grid members so that heat produced in the grids may be readily dissipated by air flowing, forced or otherwise, over the elements. Stress due to repeated expansion and contraction of the heated resistance elements can often cause failure of the elements. It is imperative that the expansion of the elements take place in a controlled manner so that the parallel alignment of the elements remains intact and uniform to avoid failure.
The normal continuous operating temperature for high capacity resistors is 375.degree. C. according to industry standards. For short intervals, the temperature of the resistor can reach higher values due to surges. In order to accommodate the high current and attendant high temperatures, it is desirable to put the maximum amount of resistor material in the smallest space while still permitting maximum air flow and cooling.
A review of the prior art illustrates low current and high current resistors as well as problems associated with high current resistors and attempts to solve such problems. U.S. Pat. No. 1,184,706 issued to Keller illustrates a multi-pass resistor element formed in a grid with provision for serial or parallel combinations. The grid elements are mounted on insulating bushings and the grids may be insulated from each other by means of insulating washers. Weinberg, U.S. Pat. No. 3,212,045, discloses a multi-stage insulating scheme for a grid-type resistor. Dubois, U.S. Pat. No. 2,969,516 illustrates a zigzag resistor grid formed from a single piece of resistance material. The grid mounting clips engage folded outboard ends of the grid for achieving a sliding joint. The grid elements are angled to increase strength. Angling of the ribs can create stress points which makes the element more susceptible to deforming and failure. Dubois, Jr., U.S. Pat. No. 3,550,058 discloses an electrical resistor structure with individual curved resistor elements which creates turbulent air flow over the elements.
Harkness, U.S. Pat. No. 4,654,627 illustrates a resistor grid assembly in which relatively wide slot resistor elements having offset ends are joined together in a series arrangement. Each element has a longitudinal upraised strengthening rib. This rib would be susceptible to deforming and failure upon repeated heating and cooling. An Australian Patent, No. 113,836 issued to Godfrey illustrates a single grid resistor element having a strengthening rib and offset ends. With the exception of the Harkness and Dubois, Sr. patents, all of the above references are not suited for high wattage requirements. None of the resistors disclosed in the references would be rated above 600 volts without standoff insulators.